There are some organizations carrying Arabidopsis T-DNA (or other methods) knockout lines for ordering. You may look into their products to see anything you want. So you can save some time to do it yourself.

You may also use current technologies such as ZFN, TALENs or CRISPR-Cas9 for your specific gene knockout. CRISPR is the most-recent developed technology and is considered the most versatile one.

In case you decide to order seeds, the European Arabidopsis Stock Centre (arabidopsis.info) is a good place to start. Using the search tools, you should be able to find mutants (and whether they are available or not) just by typing in the AGI code for your gene. However, you cannot be certain that the seeds you get are ready to use. You should check for example that they contain the T-DNA insertion in the right place and that they are homozygous knockouts.

http://arabidopsis.info/

When you can not find interest insertion line in the general seed stock website (TAIR, NASC, GABI-kat et al.), CRISP/CAS9 will satisfy your need. 

Good luck!